1. Field of the Invention
The present invention relates to an electronic apparatus, such as a laptop personal computer, that is capable of operating with a battery (secondary battery). The present invention also relates to a charging method for an electronic apparatus and to a battery used for an electronic apparatus.
2. Description of the Related Art
Lithium ion batteries widely used for mobile electronic apparatus such as laptop personal computers have a characteristic of which the internal voltage of a battery cell varies depending on the charge amount of the battery (remaining charge amount). Conversely, as the battery is charged, the voltage of the battery cell side rises. For example, when the charge amount is 100% in a battery cell having a nominal voltage of 4.2 V, the voltage of the battery cell side is 4.2 V. When the charge amount is 80%, the voltage of the battery cell becomes 4.0 V. When the charge amount is 50%, the voltage of the battery cell side becomes 3.7 V. In such a manner, the voltage of the battery cell side drops.
On the other hand, lithium ion batteries have a characteristic of which the higher the voltage of a battery cell is, the more easily a cathode active material that composes the battery cell elutes, namely the more remarkably the performance of the battery cell deteriorates. In other words, when a lithium ion battery that has been charged at a high cell voltage is not used for a long period of time, the battery performance deteriorates, namely capacity deterioration progresses. This means that when a lithium ion battery that has been charged at a high cell voltage is not used for a long period of time, the life of the battery becomes short. Thus, when the charge amount of a battery is limited to for example 80% or 50% of its maximum charge amount, the battery can be prevented from becoming the capacity deterioration.
More specifically, it is assumed that the user of a laptop personal computer has two batteries and their capacities are different and that the battery having a large capacity is called the L battery and the other battery having a small capacity is called the S battery. When the maximum charge amount of the battery has been fixed on the personal computer side, it is difficult to satisfy a user's preference of which “the charge amount of the L battery is limited to 80% of its maximum charge amount so as to suppress its capacity deterioration, whereas the charge amount of the S battery is set to 100% so as to prioritize the operation period of time of the battery”.
Such a lithium ion battery has a characteristic of which the higher the charge/discharge rate (C) is, the more the charge/discharge cycle characteristic deteriorates. On the other hand, when the charge rate is high, the charge period becomes short. Conversely, when the charge rate is low, the charge period becomes long. In this specification, the charge/discharge rate (C) means the ratio of a charge/discharge current to the nominal current capacity of a battery. 1 C corresponds to the current amount of the nominal current capacity. For example, when the nominal current capacity of a battery is 2600 mAh, 0.5 C corresponds to 1300 mA. The charge/discharge cycle characteristic means a phenomenon or a ratio of which when a battery is repeatedly charged and discharged, the battery capacity deteriorates. Thus, it is common that taking account of the charge period and the charge/discharge cycle characteristic, a proper charge rate is set to the battery.
The main body of a personal computer may support a plurality of types of batteries having different pack capacities that differ in the numbers of parallel cells and/or cell capacities. In this case, it is common that the charge current that flows in the charging circuit is fixed on the main body of the personal computer.
However, when a charge current that is optimum for a battery having a large pack capacity is caused to flow in a battery having a small pack capacity, the charge rate of the battery becomes high. Thus, because of the foregoing reason, the battery life may become short. Conversely, when a charge current that is optimum for a battery having a small pack capacity is caused to flow in a battery having a large pack capacity, the charge period of time may become too long and/or the charge completion may not be detected. In these cases, such abnormalities are likely to occur.
To solve such problems, a technique of changing the charge amount of a battery with a switch disposed on the main body side has been disclosed for example in Japanese Patent Application Laid-Open No. 2002-78222, paragraph 0006, FIG. 3 (hereinafter this document may be referred to as patent document 1). It may be contemplated that such a technique is applied for a charge current.